As wireless communications technologies develop, higher requirements are imposed on a system throughput and a system rate, and an MIMO (multiple-input multiple-output) technology emerges. In the MIMO technology, a spatial feature can be fully used to increase a system capacity without increasing transmit power and bandwidth. In addition, the MIMO technology plays an important role in increasing a peak rate and improving reliability of data transmission, expanding coverage, suppressing interference, and increasing the system capacity and the system throughput. As requirements on a rate and spectral efficiency constantly increase, enhancement and optimization of the MIMO technology are always an important direction for LTE system evolution.
A 1D antenna is used in a conventional 2D MIMO technology. To be specific, the antenna is placed in only a horizontal direction. As shown in FIG. 1A and FIG. 1B, a fixed downtilt is used for all terminals, and a beam direction can be adjusted in only the horizontal direction. As technologies develop, a 2D antenna is introduced, that is, a 3D MIMO technology. As shown in FIG. 1C, FIG. 1D, FIG. 1E, and FIG. 1F, a beam direction of the 2D antenna can be adjusted in both the horizontal direction and a vertical direction, and beam direction adjustment is freer.
In the 3D MIMO technology, an expression form of W1 is shown in Formula 1.
                                          W            1                    =                      [                                                                                                                              X                        ~                                            1                                        ⊗                                                                  X                        ~                                            2                                                                                        0                                                                              0                                                                                                                    X                        ~                                            1                                        ⊗                                                                  X                        ~                                            2                                                                                            ]                          ,                            (                  Formula          ⁢                                          ⁢          1                )            where {tilde over (X)}1 and {tilde over (X)}2 are vectors in different dimensions or vector combinations. In this case, when feeding back a W1-related PMI to a base station, a terminal needs to feed back PMIs (Precoding Matrix Indicators, precoding matrix indicator) in two dimensions. However, currently, there is no CSI measurement and feedback method specific to the 3D MIMO technology.